But the judges allowed a sentence appeal by Blackman, reducing his 10-year minimum term to eight years - the least he must serve before he can be considered for parole.

After being convicted last November at a court martial in Bulford, Wiltshire, the 39-year-old was sentenced to life with the minimum term of 10 years.

He was also "dismissed with disgrace" from the Royal Marines after he had served with distinction for 15 years, including tours of Iraq, Afghanistan and Northern Ireland.

As well as challenging his "unsafe" conviction at a recent appeal hearing, his defence argued that his sentence was "manifestly excessive".

The killing happened in Helmand province in 2011 while Blackman, who is known as Al, was serving with Plymouth-based 42 Commando.

He shot the Afghan, who had been seriously injured in an attack by an Apache helicopter, in the chest at close range with a 9mm pistol before quoting a phrase from Shakespeare as the man convulsed and died in front of him.

Blackman told him: "There you are. Shuffle off this mortal coil, you c***. It's nothing you wouldn't do to us."

He then turned to comrades and said: "Obviously this doesn't go anywhere, fellas. I just broke the Geneva Convention."

During the trial Blackman was known as Marine A, and his junior colleagues - who were both cleared - as Marines B and C.

They were later named as Corporal Christopher Watson and Marine Jack Hammond.

The shooting was captured on a camera mounted on the helmet of Cpl Watson.

Blackman, who denied murder, said he believed the victim was already dead and he was taking out his anger on a corpse.

He has said he felt ashamed at his lack of self-control, describing it as a "stupid lack of self-control and lapse in judgment".

During the appeal Blackman's QC Anthony Berry said he was convicted by a seven-man court martial board, and in explaining a point of law raised in the challenge said: "The appellant submits that the possibility that he was convicted by a simple majority renders his conviction inherently unsafe."

He argued that "by virtue of the possibility that he was convicted by a simple majority of a seven-man board there remains doubt as to whether the prosecution in fact satisfied the criminal standard of proof".

But the appeal conviction was dismissed by the three judges.

Lord Thomas said that the court martial ought to have accorded greater weight as a mitigating factor to the combat stress from which Blackman was suffering which arose out of the nature of the insurgency in Afghanistan, the way his tour of duty was arranged and other matters personally affecting him.

And, he added, the particular circumstances of the court martial did not require the additional term by way of deterrence, given the very public proceedings, the worldwide publicity and the life sentence imposed.

An MoD spokesman said: " We respect the authority and decision of the appeal court, and it would be inappropriate for us to comment on the sentence."